Grain oriented electrical steel sheet having specular surface

ABSTRACT

In order to provide a very low iron loss, it is necessary to render the surface of a steel sheet smooth (specular). In the present invention, this is effected in a finish annealing furnace to simultaneously attain a high magnetic flux density and a specular surface. Specifically, after the completion of decarburization annealing, a steel material is pickled to remove an oxide layer present on the surface of the steel sheet, coated with an annealing separator comprising a substance nonreactive or less reactive with SiO 2  and then subjected to finish annealing to provide a grain oriented electrical steel sheet having a specular surface. Magnetic domain division and tension coating of the steel sheet can provide a low iron loss value. In the finish annealing, since no time is required for dehydration, the annealing time can be shortened.

TECHNICAL FIELD

The present invention relates to a process for producing aunidirectionally grain oriented silicon steel sheet that is utilizedmainly as an iron core of transformers and other electrical equipment.In particular, the present invention aims at an improvement in the ironloss property through effective finishing of the surface of aunidirectionally grain oriented silicon steel sheet.

BACKGROUND ART

Unidirectionally grain oriented silicon steel sheets are used inmagnetic iron core in many types of electrical equipment. Theunidirectionally grain oriented silicon steel sheets are steel sheetshaving an Si content of 0.8 to 4.8% and, in the form of a product, ahighly integrated {110}<001> grain orientation.

They are required to have a high magnetic flux density (a valuerepresented by a B8 value) and a low iron loss (a value represented by aW_(17/50) value) as magnetic properties. Particularly, in recent years,there is an ever-increasing demand for a reduction in the power lossfrom the viewpoint of energy saving.

In order to comply with this demand, a technique for dividing magneticdomains has been developed as means for reducing the iron loss ofunidirectionally grain oriented silicon steel sheets.

In the case of laminated cores, for example, Japanese Unexamined PatentPublication (Kokai) No. 58-26405 discloses a method of domain refinementwherein a steel sheet after finish annealing is irradiated with a laserbeam to give a small local strain to the steel sheet, thereby dividingmagnetic domains to reduce the iron loss. On the other hand, in the caseof wound iron cores, for example, Japanese Unexamined Patent Publication(Kokai) No. 62-8617 discloses a method which enables the disappearanceof the effect of division of magnetic domains to be prevented even whenstrain release annealing (stress release annealing) is effected afterthe steel sheet is fabricated into an iron core. The iron loss has beensignificantly reduced through division of magnetic domains by theabove-described technical means.

However, observation of the migration of these magnetic domains hasrevealed that some magnetic domains do not migrate, and it has beenfound that, in addition to the division of magnetic domains, theelimination of the pinning effect, which inhibits the migration of themagnetic domains and is derived from a glass film present on the surfaceof the steel sheet, is important to a further reduction in the iron lossvalue of the unidirectionally grain oriented silicon steel sheet.

For this purpose, it is useful to prevent the formation of a glass filmon the surface of the steel sheet which inhibits migration of themagnetic domain. For example, U.S. Pat. No. 3,785,882 discloses a methodwherein a coarse high-purity alumina is used as an annealing separatorto prevent the formation of a glass film. In this method, however,inclusions just under the surface cannot be eliminated, so that theimprovement in the iron loss is 2% at the highest in terms of W_(15/60).

Further, an enhancement in the orientation integration of the materialis useful for improving the iron loss. In this connection, Taguchi andSakakura (Japanese Examined Patent Publication (Kokoku) No. 40-15644),Komatsu et al. (Japanese Examined Patent Publication (Kokoku) No.62-45285), etc. disclose methods wherein a nitride of Al is used as aninhibitor. When the method disclosed in U.S. Pat. No. 3,785,882 whereinalumina is used as the annealing separator is applied to these methodswherein a nitride of Al is used as the inhibitor, the secondaryrecrystallization becomes so unstable that it is impossible to attain animprovement in the iron loss on a commercial scale.

On the other hand, in order to regulate the inclusion just under thesurface and, at the same time, to attain a specular surface, forexample, Japanese Unexamined Patent Publication (Kokai) No. 64-83620discloses a method wherein chemical polishing or electropolishing iseffected after the completion of finish annealing. Although chemicalpolishing, electropolishing and other polishing are feasible for workingof a small amount of a sample material on a laboratory level, thepractice of these methods on a commercial scale has large problems ofthe control of concentration of chemicals, control of temperature,provision of pollution control facilities, etc., so that these methodshave not been put to practical use.

DISCLOSURE OF THE INVENTION

An object of the present invention is to solve, based on the method forthe prevention of a glass film (see for example, U.S. Pat. No.3,785,882), problems of (1) unstable secondary recrystallization of highmagnetic flux density materials using a nitride of Al as an inhibitor inconnection with Taguchi and Sakakura (Japanese Examined PatentPublication (Kokoku) No. 40-15644), Komatsu et al. (Japanese ExaminedPatent Publication (Kokoku) No. 62-45285), etc. and (2) the presence ofinclusions just under the surface of the steel sheet.

The present inventors have conducted an investigation on the cause ofunstable secondary recrystallization of high magnetic flux densitymaterials using a nitride of Al as an inhibitor with respect to theproblem (1) in connection with Taguchi and Sakakura (Japanese ExaminedPatent Publication (Kokoku) No. 40-15644) and Komatsu et al. (JapaneseExamined Patent Publication (Kokoku) No. 62-45285). As a result, theyhave found that, when the formation of a glass film is prevented, theinhibitor is rapidly weakened during finish annealing, which iscausative of the unstable secondary recrystallization. This is becausethe absence of a glass film causes nitrogen in a solid solution form toeasily come out of the system. Accordingly, the present inventors havemade various studies on means for inhibiting denitriding and, as aresult, have found that the formation of a silica film serving as abarrier to nitrogen or the enrichment of a surface segregation elementon the surface of the steel sheet are useful for this purpose.

Further, they have made studies on the problem (2), that is, theregulation of inclusions just under the surface and, as a result, havefound that an oxide layer formed in the step of decarburizationannealing has a great influence on the inclusions. As a result ofvarious studies on the method for rendering the inclusions absent, theyhave found the removal of the oxide layer on the surface of the steelsheet as decarburized is very effective and can contribute to asignificant improvement in the iron loss.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between magnetic fluxdensity B8 and the iron loss W_(17/50) of the products produced undervarious conditions;

FIG. 2 is a diagram showing the influence of an atmospheric gas on thebehavior of a change in an inhibitor (the nitrogen content) duringfinish annealing;

FIG. 3 is a GDS (glow discharge spectroscopy) chart showing the degreeof enrichment of silica on the surface of the steel sheet in finishannealing at 900° C.;

FIG. 4 is a diagram showing an influence of a surface segregationelement (Sn) on the magnetic flux density (secondary recrystallizationstability); and

FIG. 5 is a diagram showing the influence of a surface segregationelement (Sn) on the behavior of a change in an inhibitor (the nitrogencontent) during finish annealing.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described.

The present inventors have prepared two types of decarburized samples(A/B) having a sheet thickness of 0.23 mm and different from each otherin the inhibitor. Sample A is a steel sheet sample described in JapaneseExamined Patent Publication (Kokoku) No. 30-3651 wherein MnS is used asa main inhibitor, and sample B is a steel sheet sample described inJapanese Unexamined Patent Publication (Kokai) No. 62-45285 wherein anitride of Al (Al, Si)N is used as a main inhibitor.

Part of the samples, as such, were laminated using alumina as anannealing separator. On the other hand, other part of the samples werepickled to remove the oxide layer formed in the decarburizationannealing and then laminated using alumina as an annealing separator.

These laminated samples were subjected to finish annealing in two typesof annealing cycles (S1/S2). In S1, annealing was effected in a hydrogenatmosphere having a dew point of -40° C. or below. On the other hand, inS2, annealing was effected in a mixed gas comprising 75% of N₂ and 25%of H₂ in such a manner that, in order to form a silica film on thesurface of the steel sheet, the samples was heated to 800° C. at a dewpoint of 10° C. and then to 1,200° C. at a temperature rise rate of 15°C./hr. Thereafter, the samples were annealed in a H₂ gas for 20 hr toeffect purification with respect to S, N, etc.

The products thus produced were subjected to a tension coatingtreatment, a magnetic domain refinement treatment with laser beamirradiation, and magnetic properties were measured.

The results are provided in Table 1 and FIG. 1.

                  TABLE 1                                                         ______________________________________                                        Decar-                                                                        burized                      Magnetic Properties                              Sheet               Finish   (average value)                                  No.   Sample  Pickling  Annealing                                                                            B8(T)  W.sub.17/50 (W/kg)                      ______________________________________                                        1     A       Not done  S1     1.86   0.97                                    2                       S2     1.87   0.95                                    3     A       Done      S1     1.87   0.85                                    4                       S2     1.87   0.86                                    5     B       Not done  S1     1.65*  >1.5                                    6                       S2     1.93   0.73                                    7     B       Done      S1     1.68*  >1.5                                    8                       S2     1.94   0.63                                    ______________________________________                                         Note                                                                          *Secondary recrystallization undeveloped                                 

From these results, it is apparent that:

(1) in the sample A wherein MnS is used as a main inhibitor, thesecondary recrystallization is stable under all the conditions (B8:about 1.86T), whereas in the sample B wherein a nitride of Al is used asa main inhibitor, the secondary recrystallization occurs to provide aproduct having a high flux density (B8: about 1.93T) only when use ismade of the finish annealing cycle S2 wherein a silica film is formed onthe surface of the steel sheet before the secondary recrystallization;and

(2) in both the samples A and B, an about 0.1 W/kg improvement in theiron loss can be attained by pickling the decarburized steel sheet toremove an oxide film formed in the decarburization annealing.

The results of an examination on a change in the inhibitor (the nitrogencontent) for finish annealing cycles S1 and S2 are shown in FIG. 2. Whenthe S1 cycle is compared with a conventional technique where MgO iscoated in a water slurry form to form a glass film, it is apparent that,in the S1 cycle, nitrogen rapidly decreases at a temperature of about1,000° C. at which the secondary recrystallization develops. On theother hand, as shown in FIG. 3, in the S2 cycle wherein a silica film isformed on the surface of the steel sheet, it is apparent that, as withthe results of the conventional technique, the steel sheet gives rise tono reduction in nitrogen content until the temperature reaches atemperature range of from 1,000° to 1,100° C. in which therecrystallization structure develops with the inhibitor remainingstable. Thus, the secondary recrystallization can be stabilized toprovide products having a high magnetic flux density by regulating thesurface of the steel sheet to prevent the denitriding for the purpose ofstably maintaining the inhibitor. The iron loss was reduced by about 0.2W/kg (20%) by improving the magnetic flux density.

In the samples wherein the oxide layer formed in the decarburizationanneal has not been removed, fine inclusions are present just under thesurface of the samples. These inclusions are not observed in sampleswherein the oxide layer formed in the decarburization annealing has beenremoved by pickling, and, as is apparent from Table 1, an about 0.1 W/kg(10%) reduction in the iron loss (W_(17/50)) value can be attained byadopting the pickling.

As is apparent from the foregoing description, the iron loss value ofthe product can be improved (1) by about 20% by regulating the inhibitorto improve the magnetic flux density of the steel sheet and (2) by about10% by removing the oxide layer of the decarburized steel sheet toeliminate inclusions present just under the surface. Further, acombination of these two techniques enables the iron loss value to beimproved by about 30%.

Embodiments of the present invention will now be described.

The magnetic flux density of the steel sheet can be enhanced by applyinga production process proposed by Taguchi, Sakakura et al. wherein AlNand MnS are used as the main inhibitor (see, for example, JapaneseExamined Patent Publication (Kokoku) No. 40-15644) or a productionprocess proposed by Komatsu et al. wherein (Al, Si) N is used as themain inhibitor (see, for example, Japanese Examined Patent Publication(Kokoku) No. 62-45285). In this case, as described above, the preventionof denitriding on the surface of the steel sheet to stabilize theinhibitor comprising a nitride of Al is indispensable.

In order to prevent the denitriding, it is useful to effect, prior tothe development of secondary recrystallization, (1) the formation of asilica film on the surface of the steel sheet or (2) the enrichment ofsurface segregation elements, such as Sn, Sb and Pb, on the surface ofthe steel sheet.

The atmosphere gas just above the steel sheet in a temperature range offrom 600° to 900° C. used until the secondary recrystallization developsin the finish annealing may be rendered weakly oxidizing relative to Si(degree of oxidization pH₂ O/pH₂ : 0.01 to 0.1) for the purpose offorming a silica film on the surface of the steel sheet. In this rangeof degrees of oxidization, a uniform oxide film can be formed byexternal oxidization of Si contained in the steel to prevent thepermeation of nitrogen through the film. When the degree of oxidizationis excessively low, the time taken for the silica film to be formedbecomes excessively long, which is unfavorable from the practicalviewpoint. On the other hand, when the degree of oxidization isexcessively high, since a nonuniform silica layer is formed due tointernal oxidization, it becomes impossible to prevent the permeation ofnitrogen through the film.

The enrichment of surface segregation elements, such as Sn, Sb and Pb,on the surface of the steel sheet is also useful for preventingdenitriding. In the samples wherein these surface segregation elementsare enriched on the surface of the steel sheet, denitriding duringfinish annealing can be prevented, which enables the inhibitor to remainstable until the temperature reaches a high temperature, so that thesecondary recrystallized structure can be stably developed. Thesesurface segregation elements may be enriched on the surface of the steelsheet before the secondary recrystallization in the finish annealing. Inthis case, as described above, these elements may be added to a moltensteel or may be coated in the form of a simple substance or a compoundon the steel sheet in a stage before the finish annealing.

The influence of addition of Sn will now be described as an example withrespect to the enrichment of the surface segregation element on thesurface of the steel sheet. Silicon steel slabs comprising, in terms ofby weight, 3.3% of Si, 0.14% of Mn, 0.05% of C, 0.007% of S, 0.028% ofacid soluble Al, 0.008% of N and 0.005 to 0.3% of Sn were hot-rolledinto steel sheets having a thickness of 1.6 mm. The hot-rolled sheetswere annealed at 1,100° C. for 2 min and cold-rolled into steel sheetshaving a final thickness of 0.15 mm. The cold-rolled steel sheets weresubjected to annealing serving also as decarburization in a moist gas at850° C. for 70 sec to effect primary recrystallization.

These samples were coated with an annealing separator composed mainly ofalumina by electrostatic coating and then subjected to finish annealing.

The finish annealing was effected in an atmosphere of 100% N₂ at atemperature rise rate of 15° C./hr until the temperature reached 1,200°C. When the temperature reached 1,200° C., the atmosphere was switchedto an atmosphere of 100% of H₂ and purification annealing was theneffected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment, a magneticdomain division treatment with laser beam irradiation and measurement ofmagnetic properties. The results are shown in FIG. 4.

As is apparent from FIG. 4, in samples wherein Sn has been added in anamount of 0.03 to 0.15%, the secondary recrystallization could be stablyeffected. The reason why the recrystallization becomes unstable when theamount of addition of Sn is 0.15% or more is believed to be that thesecondary recrystallization temperature becomes excessively high.

As opposed to the conventional technique, when no water slurry is usedas the annealing separator, the deterioration in the inhibitors (such asAlN and (Al, Si)N) occurs due to denitriding from the surface.Therefore, in the material wherein Sn has been added, the formation of alayer enriched in Sn on the surface of the steel sheet can reduce therate of escape of nitrogen. A change in the N content during finishannealing is shown in FIG. 5. From FIG. 5, it is apparent that theeffect of inhibiting the denitriding can be attained by adding Sn.

The oxide layer formed in the decarburization annealing can be removedby any of a chemical method, such as pickling, or a physical method,such as mechanical grinding. In general, since the thickness of thedecarburized steel sheet is as small as 0.1 to 0.5 mm, pickling isconsidered convenient for industrial scale.

The annealing separator may be a substance nonreactive or less reactivewith silica present on the surface of the steel sheet. Examples ofmethods useful for using the annealing separator include (1) one whereina powder of Al₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO or Mg₂ SiO₄ is used byelectrostatic coating or the like in such a state that no water ofhydration is carried in the system, (2) one wherein use is made of asteel sheet having a surface layer, such as Al₂ O₃, SiO₂, ZrO₂, BaO,CaO, SrO or Mg₂ SiO₄, and (3) one which comprises preparing a waterslurry of a powder of Al₂ O₃, SiO₂, ZrO₂, SrO or Mg₂ SiO₄ having anaverage particle diameter of 0.5 to 10 μm, coating the slurry on thesurface of the steel sheet and drying the steel sheet to remove water ofhydration. When the annealing separator is used in the form of a waterslurry, if the particle diameter is larger than 10 μm, coarse particlesbite into the steel sheet, whereas if the particle is smaller than 0.5μm, seizing occurs in the steel sheet due to the activity of theparticles.

The product after finish annealing is subjected to a tension coatingtreatment and a magnetic domain division treatment such as laser beamirradiation.

The present invention will now be described with reference to thefollowing Examples.

EXAMPLES

Example 1

A hot-rolled silicon steel strip comprising 3.3% by weight of Si, 0.025%by weight of acid soluble Al, 0.009% by weight of N, 0.07% by weight ofMn, 0.015% by weight of S, 0.08% by weight of C and 0.015% by weight ofSe with the balance consisting of Fe and unavoidable impurities wasannealed at 1,120° C. for 2 min, and cold-rolled into a steel sheethaving a thickness of 0.23 mm.

The cold-rolled steel sheet was subjected to annealing serving also asdecarburization in an annealing furnace having a moist atmosphere (dewpoint: 65° C.) at 850° C. for 2 min to effect primary recrystallization.

Thereafter, the steel sheet was 1 transferred to the next step or 2pickled with a mixed solution comprising 0.5% hydrofluoric acid and 5%sulfuric acid. The two types of materials were coated with a waterslurry of Al₂ O₃ having an average particle diameter of 4.0 μm. Forcomparison, the steel sheet was 3 subjected to no pickling and thencoated with an annealing separator composed mainly of a MgO in the formof a water slurry.

These three types of materials were subjected to finish annealing in twotypes of cycles. In one cycle (S1), the materials were heated at atemperature rise rate of 15° C./hr to 1,200° C. in an atmospherecomprising 15% of N₂ and 85% of H₂ and having a degree of oxidization of0.001 or less. On the other hand, in the other cycle (S2), the materialswere heated at a temperature rise rate of 15° C./hr to 1,200° C. in anatmosphere comprising 15% of N₂ and 85% of H₂ and having a degree ofoxidization of 0.05. After the temperature reached 1,200° C., theatmosphere was switched to an atmosphere consisting of 100% hydrogen,and the materials were held at that temperature for 20 hr. After thecompletion of the finish annealing, the materials were subjected to atension coating treatment with an agent comprising phosphoric acid andchromic acid and then subjected to laser beam irradiation. Properties ofthe resultant products are given in Table 2.

                  TABLE 2                                                         ______________________________________                                                                  Magnetic                                                                      Flux                                                       Finish             Density                                                                              Iron Loss                                    Annealing                                                                            Annealing          (B8)   W.sub.17/50                                                                          Re-                                   Separator                                                                            Cycle    Surface   (tesla)                                                                              (W/kg) marks                                 ______________________________________                                        1      S1       Specular  1.68*  >1.5   Comp. Ex.                                             surface                                                              S2       Specular  1.95   0.72   Inven-                                                surface                 tion                                  2      S1       Specular  1.71*  >1.5   Comp. Ex.                                             surface                                                              S2       Specular  1.94   0.63   Inven-                                                surface                 tion                                  3      S1       Glass     1.92   0.77   Comp. Ex.                                             film                                                                 S2       Glass     1.91   0.78   Comp. Ex.                                             film                                                          ______________________________________                                         Note                                                                          *Secondary recrystallization undeveloped                                 

Example 2

A 1.4 mm-thick hot-rolled silicon steel sheet comprising 3.3% by weightof Si, 0.029% by weight of acid soluble Al, 0.008% by weight of N, 0.12%by weight of Mn, 0.007% by weight of S and 0.05% by weight of C with thebalance consisting of Fe and unavoidable impurities was annealed at1,100° C. for 2 min, and cold-rolled into a steel sheet having athickness of 0.15 mm.

The cold-rolled steel sheet was subjected to annealing serving also asdecarburization in an annealing furnace having a moist atmosphere at840° C. for 2 min to effect primary recrystallization. In order tostabilize the secondary recrystallization, the annealed steel sheet wasthen nitrided in an ammonia atmosphere to a total nitrogen content of190 ppm, thereby strengthening the inhibitor.

Thereafter, the oxide layer formed on the surface of the steel sheet wasremoved with a mixture of sulfuric acid with hydrofluoric acid, and thesteel sheet was 1 coated with Al₂ O₃ having an average particle diameterof 2.0 μm as an annealing separator by electrostatic coating, 2subjected to thermal spray with Al₂ O₃ as an annealing separator, 3coated with a water slurry of Al₂ O₃ having an average particle diameterof 2.0 μm as an annealing separator to form a coating which was thendried, and, for comparison purpose, 4 coated with MgO in the form of awater slurry (a conventional method)

These three types of materials were heated at a temperature rise rate of10° C./hr to 1,200° C. in an atmosphere gas consisting of 100% of N₂.After the temperature reached 1,200° C., the atmosphere was switched toan atmosphere consisting of 100% hydrogen, and the materials were heldat that temperature for 20 hr. After the completion of the finishannealing, the materials were subjected to a tension coating treatmentwith an agent comprising phosphoric acid and chromic acid and thensubjected to laser beam irradiation to effect magnetic domain division.Properties of the resultant products are given in Table 3.

                  TABLE 3                                                         ______________________________________                                                Surface    Magnetic                                                           Appearance Flux                                                               After      Density   Iron Loss                                        Annealing                                                                             Finish     (B8)      W.sub.17/50                                      Separator                                                                             Annealing  (tesla)   (w/kg)  Remarks                                  ______________________________________                                        1       Smooth surface                                                                           1.95      0.51    Invention                                        (Specular                                                                     surface)                                                              2       Smooth surface                                                                           1.94      0.52    Invention                                        (Specular                                                                     surface)                                                              3       Smooth surface                                                                           1.94      0.53    Invention                                        (Specular                                                                     surface)                                                              4       Glass Film 1.93      0.67    Comp. Ex.                                ______________________________________                                    

Example 3

A silicon steel slab comprising, in terms of by weight, 3.3% of Si,0.12% of Mn, 0.05% of C, 0.007% of S, 0.026% of acid soluble Al, 0.008%of N and 0.01% of Pb was heated to 1,150° C. and hot-rolled into a steelsheet having a thickness of 1.8 mm. The hot-rolled steel sheet wasannealed at 1,100° C. for 2 min and then cold-rolled into a steel sheethaving a final thickness of 0.2 mm. The cold-rolled steel sheet wassubjected to annealing serving also as decarburization in a moistatmosphere at 850° C. for 70 sec to effect primary recrystallization.Thereafter, the steel sheet was annealed in an ammonia atmosphere at750° C. to increase the nitrogen content to 0.02%, thereby strengtheningthe inhibitor. Thereafter, the steel sheet was pickled to remove theoxide layer formed on the surface of the steel sheet. (1) Part of thissteel sheet was coated with a water slurry of alumina having an averageparticle diameter of 1 μm, while (2) the other part of the steel sheetwas coated with a water slurry of magnesia. They were put on top ofanother and then subjected to finish annealing.

The finish annealing was effected in an atmosphere gas consisting of100% N₂ at a temperature rise rate of 10° C./hr until the temperaturereached 1,200° C. When the temperature reached 1,200° C., the atmospherewas switched to one consisting of 100% H₂ and purification annealing waseffected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 4.

                  TABLE 4                                                         ______________________________________                                                Magnetic                                                                      Flux          Iron Loss                                               Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)   Remarks                                        ______________________________________                                        1       1.93          0.62     Invention                                      2       1.93          0.71     Comp. Ex.                                      ______________________________________                                    

It is apparent that coating of alumina can provide an about 10%reduction (improvement) in the iron loss value as compared with coatingof magnesia in the form of a water slurry.

Example 4

A silicon steel slab comprising, in terms of by weight, 3.2% of Si,0.08% of Mn, 0.08% of C, 0.025% of S, 0.025% of acid soluble Al, 0.009%of N and 0.008% of Pb was heated to 1,320° C. and hot-rolled into asteel sheet having a thickness of 1.8 mm. The hot-rolled steel sheet wasannealed at 1,050° C. for 2 min and then cold-rolled into a steel sheethaving a thickness of 0.20 mm. The cold-rolled steel sheet was subjectedto annealing serving also as decarburization in a moist gas at 850° C.for 90 sec to effect primary recrystallization. Thereafter, (A) part ofthe steel sheet was pickled to remove the oxide layer formed on thesurface of the steel sheet, while (B) other part of the steel sheet, assuch, was coated with a water slurry of alumina having an averageparticle diameter of 1.0 μm to form a coating which was then dried. Theywere then subjected to finish annealing.

The finish annealing was effected in an atmosphere gas consisting of100% Ar at a temperature rise rate of 15° C./hr until the temperaturereached 1,200° C. When the temperature reached 1,200° C., the atmospherewas switched to an atmosphere consisting of 100% H₂ and purificationannealing was then effected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 5.

                  TABLE 5                                                         ______________________________________                                                Magnetic      Iron                                                            Flux Loss                                                             Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)  Remarks                                         ______________________________________                                        A       1.92          0.67    Invention                                       B       1.92          0.61    Invention                                       ______________________________________                                    

It is apparent that removal of the oxide layer formed in thedecarburization annealing contributes to a further improvement(reduction) in the iron loss.

Example 5

A silicon steel slab comprising, in terms of by weight, 3.3% of Si,0.12% of Mn, 0.05% of C, 0.007% of S, 0.028% of acid soluble Al, 0.008%of N and (A) 0.01%, (B) 0.05% or (C) 0.1% of Sb was heated to 1,150° C.and hot-rolled into a steel sheet having a thickness of 1.6 mm. Thehot-rolled steel sheet was annealed at 1,100° C. for 2 min and thencold-rolled into a steel sheet having a final thickness of 0.15 mm. Thecold-rolled steel sheet was subjected to annealing serving also asdecarburization in a moist gas at 830° C. for 70 sec to effect primaryrecrystallization. Thereafter, the steel sheet was annealed in anammonia atmosphere at 750° C. to increase the nitrogen content to 0.02%,thereby strengthening the inhibitor. (1) Part of this steel sheet waspickled and coated with alumina by electrostatic coating, while (2) theother part of the steel sheet was coated with a water slurry ofmagnesia. They were then subjected to finish annealing.

The finish annealing was effected in an atmosphere gas consisting of100% N₂ at a temperature rise rate of 10° C./hr until the temperaturereached 1,200° C. When the temperature reached 1,200° C., the atmospherewas switched to an atmosphere consisting of 100% H₂ and purificationannealing was then effected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 6.

                  TABLE 6                                                         ______________________________________                                                Magnetic      Iron                                                            Flux          Loss                                                    Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)  Remarks                                         ______________________________________                                        A1      1.76          --      Comp. Ex.                                       A2      1.89          0.72    Comp. Ex.                                       B1      1.93          0.55    Invention                                       B2      1.91          0.66    Comp. Ex.                                       C1      1.90          0.61    Invention                                       C2      1.90          0.69    Comp. Ex.                                       ______________________________________                                    

It is apparent that coating of alumina by electrostatic coating canprovide a reduction (an improvement) in the iron loss value over coatingof magnesia in the form of a water slurry.

Example 6

A silicon steel slab comprising, in terms of by weight, 3.2% of Si,0.08% of Mn, 0.08% of C, 0.025% of S, 0.026% of acid soluble Al, 0.009%of N and 0.1% of Sn was heated to 1,320° C. and hot-rolled into a steelsheet having a thickness of 2.3 mm. The hot-rolled steel sheet wasannealed at 1,050° C. for 2 min, cold-rolled into a steel sheet having athickness of 1.4 mm, and further annealed at 1,120° C. for 2 min.Thereafter, the annealed steel sheet was cold-rolled into a steel sheethaving a final thickness of 0.15 mm. The cold-rolled steel sheet wassubjected to annealing serving also as decarburization in a moist gas at850° C. for 90 sec to effect primary recrystallization. Thereafter, thesteel sheet was pickled to remove the oxide layer present on the surfaceof the steel sheet, and (1) part of this steel sheet was coated withalumina by electrostatic coating, while (2) other part of the steelsheet was coated with a water slurry of magnesia. They were put on topof another and then subjected to finish annealing.

The finish annealing was effected in an atmosphere gas consisting of100% Ar at a temperature rise rate of 15° C./hr until the temperaturereached 1,200° C. When the temperature reached 1,200° C., the atmospherewas switched to an atmosphere consisting of 100% H₂ and purificationannealing was then effected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 7.

                  TABLE 7                                                         ______________________________________                                                Magnetic      Iron                                                            Flux          Loss                                                    Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)  Remarks                                         ______________________________________                                        A       1.93          0.55    Invention                                       B       1.91          0.67    Comp. Ex.                                       ______________________________________                                    

Example 7

A silicon steel slab comprising, in terms of by eight, 3.3% of Si, 0.12%of Mn, 0.05% of C, 0.007% of S, 0.026% of acid soluble Al and 0.008% ofN with the balance consisting essentially of Fe and unavoidableimpurities was heated to 1,150° C. and hot-rolled into a steel sheethaving a thickness of 2.0 mm. The hot-rolled steel sheet was annealed at1,100° C. for 2 min and cold-rolled into a steel sheet having a finalthickness of 0.23 mm. The cold-rolled steel sheet was subjected toannealing, serving also as decarburization, in a moist gas at 850° C.for 70 sec to effect primary recrystallization. Then, the steel sheetwas annealed in an ammonia atmosphere at 750° C. to increase thenitrogen content to 0.02%, thereby strengthening the inhibitor.Thereafter, the steel sheet was pickled to remove the oxide layerpresent on the surface of the steel sheet. Part of the steel sheet wascoated with a powder of (A) Al₂ O₃, (B) Al₂ O₃ +Sn, (C) Al₂ O₃ +Sb, (D)Al₂ O₃ +Pb, (E) Al₂ O₃ +SnO or (F) Al₂ O₃ +PbO by electrostatic coating,while (G) other part of the steel sheet was coated with a water slurryof MgO. They were put on top of another and then subjected to finishannealing.

The finish annealing was effected in an atmosphere comprising 25% N₂ and75% H₂ at a temperature rise rate of 15° C./hr until the temperaturereached 1,200° C. When the temperature reached 1,200° C., the atmospherewas switched to an atmosphere consisting of 100% H₂ and purificationannealing was then effected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 8.

                  TABLE 8                                                         ______________________________________                                                Magnetic      Iron                                                            Flux          Loss                                                    Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)  Remarks                                         ______________________________________                                        A        1.65*        >1.5    Comp. Ex.                                       B       1.93          0.64    Invention                                       C       1.92          0.65    Invention                                       D       1.93          0.63    Invention                                       E       1.92          0.65    Invention                                       F       1.92          0.65    Invention                                       G       1.91          0.78    Comp. Ex.                                       ______________________________________                                    

It is apparent that the secondary recrystallization can be stablydeveloped by adding, as an annealing separator, a surface segregationelement or a compound of such an element and enriching the element onthe surface of the steel sheet during finish annealing.

Further, it is also apparent that coating of alumina by electrostaticcoating can provide a lower (better) iron loss value than coating ofmagnesia in the form of a water slurry.

Example 8

A silicon steel slab comprising, in terms of by weight, 3.2% of Si,0.08% of Mn, 0.08% of C, 0.08% of S, 0.025% of acid soluble Al and0.009% of N with the balance consisting essentially of Fe andunavoidable impurities was heated to 1,320° C. and hot-rolled into asteel sheet having a thickness of 2.0 mm. The hot-rolled steel sheet wasannealed at 1,050° C. for 2 min, rolled into a steel sheet having athickness of 1.4 mm and then annealed at 1,000° C. for 2 min. (A) Partof the steel sheet was plated with Sn (0.01 g/m²), while (B) the otherpart of steel sheet, as such, was further cold-rolled into a steel sheethaving a thickness of 0.14 mm. The cold-rolled steel sheet was subjectedto annealing, serving also as decarburization, in a moist gas at 850° C.for 90 sec to effect primary recrystallization. Then, the steel sheetwas pickled to remove the oxide layer present on the surface of thesteel sheet. The steel sheet was coated with a water slurry of aluminahaving an average particle diameter of 2.0 μm to form a coating whichwas then dried. The steel sheets were then subjected to finishannealing.

The finish annealing was effected in an atmosphere consisting of 100% Arat a temperature rise rate of 15° C./hr until the temperature reached1,200° C. When the temperature reached 1,200° C., the atmosphere wasswitched to an atmosphere consisting of 100% H₂ and purificationannealing was then effected at that temperature for 20 hr.

These samples were subjected to a tension coating treatment and thensubjected to laser beam irradiation to effect magnetic domain division.Magnetic properties of the resultant products are given in Table 9.

                  TABLE 9                                                         ______________________________________                                                Magnetic      Iron                                                            Flux          Loss                                                    Sample  Density       W.sub.17/50                                             No.     (B8) (T)      (w/kg)  Remarks                                         ______________________________________                                        A       1.91          0.59    Invention                                       B        1.65*        >1.5    Comp. Ex.                                       ______________________________________                                         Note)*: Secondary recrystallization undeveloped                          

Example 9

A hot-rolled silicon steel strip comprising 3.3% by weight of Si, 0.025%by weight of acid soluble Al, 0.009% by weight of N, 0.07% by weight ofMn, 0.015% by weight of S, 0.08% by weight of C, 0.015% by weight of Se,0.13% by weight of Sn and 0.07% by weight of Cu with the balanceconsisting of Fe and unavoidable impurities was annealed at 1,120° C.for 2 min, and cold-rolled into a steel sheet having a thickness of 0.20mm.

The cold-rolled steel sheet was subjected to annealing serving also asdecarburization in an annealing furnace having a moist atmosphere (dewpoint: 65° C.) at 850° C. for 2 min to effect primary recrystallization.

Thereafter, the steel sheet was 1 transferred to the next step or 2pickled with a mixed solution comprising 0.5% of hydrofluoric acid and5% of sulfuric acid. The two types of materials were coated with a waterslurry of Al₂ O₃ having an average particle diameter of 4.0 μm. Forcomparison, 3 the steel sheet was coated with an annealing separatorcomposed mainly of a MgO in the form of a water slurry without pickling.

These three types of materials were heated at a temperature rise rate of15° C./hr to 1,200° C. in an atmosphere comprising 25% N₂ and 75% H₂.After the temperature reached 1,200° C., the atmosphere was switched toan atmosphere consisting of 100% hydrogen, and the materials were heldat that temperature for 20 hr. After the completion of the finishannealing, the materials were irradiated with a laser beam and thensubjected to a tension coating treatment with an agent comprisingphosphoric acid and chromic acid. Properties of the resultant productsare given in Table 10.

                  TABLE 10                                                        ______________________________________                                        Surface                                                                       Appearance                                                                    Before                                                                        Finish              Magnetic                                                  Annealing                                                                              Surface    Flux                                                      And      Appearance Density   Iron Loss                                       Annealing                                                                              After Finish                                                                             (B8)      W.sub.13/50                                     Separator                                                                              Annealing  (tesla)   (W/kg) Remarks                                  ______________________________________                                        1        Smooth surface                                                                           1.89      0.35   Invention                                         (Specular                                                                     surface)                                                             2        Smooth surface                                                                           1.90      0.33   Invention                                         (Specular                                                                     surface)                                                             3        Glass      1.90      0.40   Comp. Ex.                                ______________________________________                                    

It is apparent that the products provided according to the process ofthe present invention exhibit a good property (a low iron loss) even ata low magnetic field (1.3 T).

Example 10

A hot-rolled silicon steel strip comprising 3.2% by weight of Si, 0.029%by weight of acid soluble Al, 0.008% by weight of N, 0.13% by weight ofMn, 0.007% by weight of S and 0.05% by weight of C with the balanceconsisting of Fe and unavoidable impurities was annealed at 1,100° C.for 2 min, and cold-rolled into a steel sheet having a thickness of 0.18mm.

The cold-rolled steel sheet was subjected to annealing, serving also asdecarburization, in an annealing furnace having a moist atmosphere at820° C. for 2 min to effect primary recrystallization. Then, in order tostabilize the secondary recrystallization, the annealed steel sheet wasnitrided in an ammonia atmosphere to a total nitrogen content of 190ppm, thereby strengthening the inhibitor.

Thereafter, the steel sheet was 1 treated with a mixture of sulfuricacid with hydrofluoric acid to remove the oxide layer formed on thesurface of the steel sheet and then coated with a water slurry of Al₂ O₃having an average particle diameter of 2.0 μm as an annealing separator,2 coated with a water slurry of Al₂ O₃ having an average particlediameter of 2.0 μm as an annealing separator, and 3 coated with a waterslurry of an annealing separator composed mainly of MgO.

These three types of materials were heated at a temperature rise rate of30° C./hr to 1,200° C. in an atmosphere comprising 25% N₂ and 75% H₂.After the temperature reached 1,200° C., the atmosphere was switched toan atmosphere consisting of 100% hydrogen, and the materials were heldat that temperature for 20 hr. After the completion of the finishannealing, the materials were irradiated with a laser beam and thensubjected to a tension coating treatment with an agent comprisingphosphoric acid and chromic acid. Properties of the resultant productsare given in Table 11.

                  TABLE 11                                                        ______________________________________                                        Surface                                                                       Appearance                                                                    Before                                                                        Finish              Magnetic                                                  Annealing                                                                              Surface    Flux                                                      And      Appearance Density   Iron Loss                                       Annealing                                                                              After Finish                                                                             (B8)      W.sub.13/50                                     Separator                                                                              Annealing  (tesla)   (W/kg) Remarks                                  ______________________________________                                        1        Smooth     1.95      0.29   Invention                                         surface                                                                       (Specular                                                                     surface)                                                             2        Smooth     1.92      0.32   Invention                                         surface                                                                       (Specular                                                                     surface)                                                             3        Glass      1.93      0.37   Comp. Ex.                                ______________________________________                                    

Example 11

A hot-rolled silicon steel strip comprising 3.2% by weight of Si, 0.030%by weight of acid soluble Al, 0.008% by weight of N, 0.13% by weight ofMn, 0.007% by weight of S and 0.05% by weight of C with the balanceconsisting of Fe and unavoidable impurities was annealed at 1,100° C.for 2 min, and cold-rolled into a steel sheet having a thickness of 0.15mm.

The cold-rolled steel sheet was subjected to annealing, serving also asdecarburization, in an annealing furnace having a moist atmosphere at820° C. for 2 min to effect primary recrystallization. In order tostabilize the secondary recrystallization, the annealed steel sheet wasthen nitrided in an ammonia atmosphere to a total nitrogen content of200 ppm, thereby strengthening the inhibitor.

Thereafter, the steel sheet was treated with a mixture of sulfuric acidand hydrofluoric acid to remove the oxide layer formed on the surface ofthe steel sheet, and then 1 coated with a water slurry of Al₂ O₃ havingan average particle diameter of 2.0 μm as an annealing separator andheated to 1,200° C. in an atmosphere consisting of 100% H₂, 2 coatedwith a water slurry of Al₂ O₃ having an average particle diameter of 2.0μm as an annealing separator and heated to 1,200° C. in an atmospherecomprising 5% of N₂ and 95% of H₂, 3 coated with a water slurry of Al₂O₃ having an average particle diameter of 2.0 μm as an annealingseparator and heated to 1,200° C. in an atmosphere comprising 75% of N₂and 25% of H₂, and, for comparison purpose, 4 coated with a water slurryof MgO as an annealing separator and heated to 1,200° C. in anatmosphere comprising 5% N₂ and 95% H₂. In each case, heating to 1,200°C. was effected at a temperature rise rate of 30° C./hr. After thetemperature reached 1,200° C., the atmosphere was switched to anatmosphere consisting of 100% hydrogen, and the materials were held atthat temperature for 20 hr.

After the completion of the finish annealing, the materials wereirradiated with a laser beam and then subjected to a tension coatingtreatment with an agent comprising phosphoric acid and chromic acid.Properties of the resultant products are given in Table 12.

                  TABLE 12                                                        ______________________________________                                        Annealing            Magnetic                                                 Separator Surface    Flux                                                     And Finish                                                                              Appearance Density   Iron Loss                                      Annealing After Finish                                                                             (B8)      W.sub.13/50                                                                          Re-                                     Atmosphere                                                                              Annealing  (tesla)   (W/kg) marks                                   ______________________________________                                        1         Smooth surface                                                                           1.92      0.31   Inven-                                            (Specular                   tion                                              surface)                                                            2         Smooth surface                                                                           1.95      0.26   Inven-                                            (Specular                   tion                                              surface)                                                            3         Smooth surface                                                                           1.96      0.25   Inven-                                            (Specular                   tion                                              surface)                                                            4         (Glass)    1.92      0.39   Comp. Ex.                                         Dull gloss                                                          ______________________________________                                    

The formation of a small amount of a glass film was observed in thematerial wherein a water slurry of MgO was used as the annealingseparator. This rendered the smoothness of the surface of the steelsheet so unsatisfactory that the magnetic properties of the steel sheetwere poor.

Example 12

A primary recrystallized steel sheet was prepared in the same manner asthat of Example 11. In order to stabilize the secondaryrecrystallization, the steel sheet was then nitrided in an ammoniaatmosphere to a total nitrogen content of 210 ppm, thereby strengtheningthe inhibitor.

Thereafter, the steel sheet was treated with a mixture of sulfuric acidwith hydrofluoric acid to remove the oxide layer formed on the surfaceof the steel sheet, and then 1 coated with alumina (Al₂ O₃) having anaverage particle diameter of 2.0 μm as an annealing separator byelectrostatic coating and heated to 1,200° C. in an atmosphereconsisting of 100% H₂, 2 coated with alumina (Al₂ O₃) having an averageparticle diameter of 2.0 μm as an annealing separator by electrostaticcoating and heated to 1,200° C. in an atmosphere comprising 5% N₂ and95% H₂, 3 coated with alumina (Al₂ O₃) having an average particlediameter of 2.0 μm as an annealing separator by electrostatic coatingand heated to 1,200° C. in an atmosphere comprising 75% N₂ and 25% H₂,and, for comparison purpose, 4 coated with a water slurry of MgO as anannealing separator and heated to 1,200° C. in an atmosphere comprising5% N₂ and 95% H₂. In each case, heating to 1,200° C. was effected at atemperature rise rate of 30° C./hr. After the temperature reached 1200°C., the atmosphere was switched to an atmosphere consisting of 100%hydrogen, and the materials were held at that temperature for 20 hr.

After the completion of the finish annealing, the materials wereirradiated with a laser beam and then subjected to a tension coatingtreatment with an agent comprising phosphoric acid and chromic acid.Properties of the resultant products are given in Table 13.

                  TABLE 13                                                        ______________________________________                                        Annealing            Magnetic                                                 Separator Surface    Flux                                                     And Finish                                                                              Appearance Density   Iron Loss                                      Annealing After Finish                                                                             (B8)      W.sub.13/50                                                                          Re-                                     Atmosphere                                                                              Annealing  (tesla)   (W/kg) marks                                   ______________________________________                                        1         Smooth surface                                                                           1.93      0.30   Inven-                                            (Specular                   tion                                              surface)                                                            2         Smooth surface                                                                           1.95      0.25   Inven-                                            (Specular                   tion                                              surface)                                                            3         Smooth surface                                                                           1.96      0.25   Inven-                                            (Specular                   tion                                              surface)                                                            4         (Glass)    1.93      0.38   Comp. Ex.                                         Dull gloss                                                          ______________________________________                                    

The formation of a small amount of a glass film was observed in thematerial wherein a water slurry of MgO was used as the annealingseparator. This rendered the smoothness of the surface of the steelsheet so unsatisfactory that the magnetic properties of the steel sheetwere poor.

Example 13

A hot-rolled silicon steel strip comprising 3.2% by weight of Si, 0.030%by weight of acid soluble Al, 0.007% by weight of N, 0.14% by weight ofMn, 0.007% by weight of S and 0.05% by weight of C with the balanceconsisting of Fe and unavoidable impurities was annealed at 1,100° C.for 2 min, and cold-rolled into a steel sheet having a thickness of 0.15mm.

The cold-rolled steel sheet was subjected to annealing, serving also asdecarburization, in an annealing furnace having a moist atmosphere at850° C. for 2 min to effect primary recrystallization. In order tostabilize the secondary recrystallization, the annealed steel sheet wasthen nitrided in an ammonia atmosphere to a total nitrogen content of200 ppm, thereby strengthening the inhibitor.

Thereafter, the steel sheet was treated with a mixture of sulfuric acidwith hydrofluoric acid to remove the oxide layer formed on the surfaceof the steel sheet, and then 1 coated with a water slurry of alumina(Al₂ O₃) having an average particle diameter of 0.3 μm as an annealingseparator, 2 coated with a water slurry of alumina (Al₂ O₃) having anaverage particle diameter of 0.5 μm as an annealing separator, 3 coatedwith a water slurry of alumina (Al₂ O₃) having an average particlediameter of 3.0 μm as an annealing separator, 4 coated with a waterslurry of alumina (Al₂ O₃) having an average particle diameter of 10.0μm as an annealing separator, 5 coated with a water slurry of alumina(Al₂ O₃) having an average particle diameter of 14.9 μm as an annealingseparator, and 6 coated with a water slurry of alumina (Al₂ O₃) havingan average particle diameter of 35 μm as an annealing separator.

These materials were heated at a temperature rise rate of 30° C./hr to1,200° C. in an atmosphere comprising 75% N₂ and 25% H₂. After thetemperature reached 1,200° C., the atmosphere was switched to anatmosphere consisting of 100% of hydrogen, and the materials were heldat that temperature for 20 hr. After the completion of the finishannealing, the materials were irradiated with a laser beam and thensubjected to a tension coating treatment with an agent comprisingphosphoric acid and chromic acid. Properties of the resultant productsare given in Table 14.

                  TABLE 14                                                        ______________________________________                                        Surface             Magnetic                                                  Appearance                                                                             Surface    Flux                                                      Before   Appearance Density   Iron Loss                                       Finish   After Finish                                                                             (B8)      W.sub.13/50                                     Annealing                                                                              Annealing  (tesla)   (w/kg) Remarks                                  ______________________________________                                        1        Alumina    1.95      0.30   Comp. Ex.                                         sintered                                                                      surface                                                              2        Smooth surface                                                                           1.95      0.26   Inven-                                            (Specular                   tion                                              surface                                                              3        Smooth surface                                                                           1.94      0.25   Inven-                                            (Specular                   tion                                              surface)                                                             4        Smooth surface                                                                           1.95      0.26   Inven-                                            (Specular                   tion                                              surface)                                                             5        Rough metallic                                                                           1.94      0.29   Comp. Ex.                                         surface                                                              6        Rough metallic                                                                           1.93      0.32   Comp. Ex.                                         surface                                                              ______________________________________                                    

When alumina having an average particle diameter of less than 0.5 μm wasused as the annealing separator, a sinter of alumina was deposited onthe surface of the steel sheet. On the other hand, when alumina havingan average particle diameter exceeding 10.0 μm was used as the annealingseparator, alumina particles bit into the steel sheet, which caused theroughness of the surface of the steel sheet to become so large that theroughness could be confirmed with a finger and the alumina present onthe surface of the steel sheet could be confirmed by observation underan electron microscope.

Example 14

A cold-rolled steel sheet was prepared in the same manner as that ofExample 11. The cold-rolled steel sheet was subjected to annealing,serving also as decarburization, in an annealing furnace having a moistatmosphere at 840° C. for 2 min to effect primary recrystallization. Inorder to stabilize the secondary recrystallization, the steel sheet wasthen nitrided in an ammonia atmosphere to a total nitrogen content of210 ppm, thereby strengthening the inhibitor. Thereafter, the steelsheet was treated with a mixture of sulfuric acid and hydrofluoric acidto remove the oxide layer formed on the surface of the steel sheet, andthen 1 coated with alumina (Al₂ O₃) having an average particle diameterof 0.3 μm as an annealing separator by electrostatic coating, 2 coatedwith alumina (Al₂ O₃) having an average particle diameter of 3.0 μm asan annealing separator by electrostatic coating, 3 coated with silicahaving an average particle diameter of 3.0 μm as an annealing separatorby electrostatic coating, 4 coated with zirconia having an averageparticle diameter of 3.3 μm as an annealing separator by electrostaticcoating, 5 coated with strontium oxide having an average particlediameter of 3.0 μm as an annealing separator by electrostatic coating,and 6 coated with forsterite having an average particle diameter of 3.0μm as an annealing separator by electrostatic coating. These materialswere heated at a temperature rise rate of 30° C./hr to 1,200° C. in anatmosphere comprising 75% of N₂ and 25% of H₂. After the temperaturereached 1,200° C., the atmosphere was switched to an atmosphereconsisting of 100% hydrogen, and the materials were held at thattemperature for 20 hr. After the completion of the finish annealing, thematerials were irradiated with a laser beam and then subjected to atension coating treatment with an agent comprising phosphoric acid andchromic acid. Properties of the resultant products are given in Table15.

                  TABLE 15                                                        ______________________________________                                                            Magnetic                                                          Surface     Flux                                                              Appearance  Density   Iron Loss                                       Annealing                                                                             After Finish                                                                              (B8)      W.sub.13/50                                     Separator                                                                             Annealing   (tesla)   (w/kg) Remarks                                  ______________________________________                                        1       Alumina     1.94      0.33   Comp. Ex.                                        sintered                                                              2       Smooth      1.94      0.27   Inven-                                           surface                      tion                                             (Specular                                                                     surface                                                               3       Smooth      1.95      0.27   Inven-                                           (Specular                    tion                                             surface)                                                              4       Smooth      1.96      0.26   Inven-                                           surface               tion                                                    (Specular                                                                     surface)                                                              5       Smooth      1.96      0.26   Inven-                                           surface                      tion                                             Specular                                                                      surface)                                                              6       Smooth      1.94      0.29   Inven-                                           surface                      tion                                             (Specular                                                                     surface)                                                              ______________________________________                                    

Industrial Applicability!

According to the present invention, a grain oriented electrical steelsheet having a surface that has little unevenness causative of theinhibition of magnetic properties, i.e., a specular surface, can beeasily provided, and a magnetic material having a very low iron loss canbe provided by subjecting the steel sheet to a laser beam irradiationtreatment for division of magnetic domains and a tension coatingtreatment. In the production of a grain oriented electrical steel sheetaccording to the present invention, since the treatment for renderingthe surface of the steel sheet specular can be very easily effected in aconventional finish annealing furnace, the present invention is veryvaluable from the viewpoint of industry.

We claim:
 1. A process for producing a grain oriented silicon steelsheet employing an aluminum nitride inhibitor comprising:providing asteel material consisting essentially of, in terms of weight percent,0.8 to 4.8% of Si, 0.012 to 0.05% of acid soluble Al, and 0.01% or lessof N with the balance being essentially Fe and unavoidable impurities;forming said steel material into a steel sheet, said steel sheet havinga surface; subjecting said steel sheet to decarburization annealing andthen nitriding after decarburization annealing; coating an annealingseparator on the surface of said steel sheet, wherein said annealingseparator is at least one member selected from the group consisting ofAl₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, and MgSiO₄, whereby said annealingseparator is disposed between adjacent steel sheet surfaces duringfinish annealing; preventing occurrence of denitriding of said steelsheet during finish annealing by forming prior to secondaryrecrystallization taking place a structure means on the surface of saidsteel sheet for preventing said occurrence of said denitriding duringfinish annealing, wherein glass film is absent from said structuremeans; finish annealing said steel sheet coated with said annealingseparator thereby providing a steel sheet having a surface with glassfilm formation being absent on said surface.
 2. A process for producinga grain oriented silicon steel sheet employing an aluminum nitrideinhibitor comprising:providing a steel material consisting essentiallyof, in terms of weight percent, 0.8 to 4.8% of Si, 0.012 to 0.05% ofacid soluble Al, 0.01% or less of N, 0.02 to 0.3% of Mn, and 0.005 to0.040% of S with the balance being essentially Fe and unavoidableimpurities; forming said steel material into a steel sheet, said steelsheet having a surface, including cold rolling said steel sheet one ormore times, with intermediate annealing effected between cold rollingsif there is more than one cold rolling; after cold rolling, subjectingsaid steel sheet to decarburization annealing; coating an annealingseparator on the surface of said steel sheet, wherein said annealingseparator is at least one member selected from the group consisting ofAl₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, and MgSiO₄, whereby said annealingseparator is disposed between adjacent steel sheet surfaces duringfinish annealing; preventing occurrence of denitriding of said steelsheet during finish annealing by forming prior to secondaryrecrystallization taking place a structure means on the surface of saidsteel sheet for preventing said occurrence of said denitriding duringfinish annealing, wherein glass film is absent from said structuremeans; finish annealing said steel sheet coated with said annealingseparator thereby providing a steel sheet having a surface with glassfilm formation being absent on said surface.
 3. A process according toclaim 1 or 2 further comprising coating at least one member selectedfrom the group consisting of Al₂ O₃, SiO₂, ZrO₂, BaO, SrO, and Mg₂ SiO₄as the annealing separator on the surface of the steel sheet, saidcoating taking place in a manner such that there is no water ofhydration in the coating.
 4. A process for producing a grain orientedsilicon steel sheet employing an aluminum nitride inhibitorcomprising:providing a steel material consisting essentially of, interms of weight percent, 0.8 to 4.8% of Si, 0.012 to 0.05% of acidsoluble Al, and 0.01% or less of N with the balance being essentially Feand unavoidable impurities; forming said steel material into a steelsheet, said steel sheet having a surface; subjecting said steel sheet todecarburization annealing and then nitriding after decarburizationannealing; coating an annealing separator on the surface of said steelsheet wherein said annealing separator is at least one member selectedfrom the group consisting of Al₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, andMgSiO₄, whereby said annealing separator is disposed between adjacentsteel sheet surfaces during finish annealing; preventing occurrence ofdenitriding of said steel sheet during finish annealing by maintainingan atmosphere during finish annealing prior to secondaryrecrystallization that is weakly oxidizing relative to Si wherein pH₂O/pH₂ is 0.01 to 0.1, thereby forming by external oxidation a SiO₂ filmon the surface of said steel sheet, thereby said SiO₂ film providing asurface structure means on the surface of said steel sheet for saidpreventing said occurrence of denitriding during finish annealing;finish annealing said steel sheet coated with said annealing separatorthereby providing a steel sheet having a surface with glass filmformation being absent on said surface.
 5. A process for producing agrain oriented silicon steel sheet employing an aluminum nitrideinhibitor comprising:providing a steel material consisting essentiallyof, in terms of weight percent, 0.8 to 4.8% of Si, 0.012 to 0.05% ofacid soluble Al, 0.01% or less of N, 0.02 to 0.3% of Mn, and 0.005 to0.040% of S with the balance being essentially Fe and unavoidableimpurities; forming said steel material into a steel sheet, said steelsheet having a surface, including cold rolling said steel sheet one ormore times, with intermediate annealing effected between cold rollingsif there is more than one cold rolling; after cold rolling, subjectingsaid steel sheet to decarburization annealing; coating an annealingseparator on the surface of said steel sheet wherein said annealingseparator is at least one member selected from the group consisting ofAl₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, and MgSiO₄, whereby said annealingseparator is disposed between adjacent steel sheet surfaces duringfinish annealing; preventing occurrence of denitriding of said steelsheet during finish annealing by maintaining an atmosphere during finishannealing prior to secondary recrystallization that is weakly oxidizingrelative to Si wherein pH₂ O/pH₂ is 0.01 to 0.1, thereby forming byexternal oxidation a SiO₂ film on the surface of said steel sheet,thereby said SiO₂ film providing a surface structure means on thesurface of said steel sheet for said preventing said occurrence ofdenitriding during finish annealing; finish annealing said steel sheetcoated with said annealing separator thereby providing a steel sheethaving a surface with glass film formation being absent on said surface.6. A process for producing a grain oriented silicon steel sheetemploying an aluminum nitride inhibitor comprising:providing a steelmaterial consisting essentially of, in terms of weight percent, 0.8 to4.8% of Si, 0.012 to 0.05% of acid soluble Al, and 0.01% or less of Nwith the balance being essentially Fe and unavoidable impurities;forming said steel material into a steel sheet, said steel sheet havinga surface; subjecting said steel sheet to decarburization annealing andthen nitriding after decarburization annealing; coating an annealingseparator on the surface of said steel sheet wherein said annealingseparator is at least one member selected from the group consisting ofAl₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, and MgSiO₄, whereby said annealingseparator is disposed between adjacent steel sheet surfaces duringfinish annealing; preventing occurrence of denitriding of said steelsheet during finish annealing by enriching prior to secondaryrecrystallization during finish annealing a surface segregation elementon the surface of said steel sheet, said surface segregation elementbeing a member selected from the group consisting of Sn, Sb and Pb,thereby said surface segregation element providing a surface structuremeans on the surface of said steel sheet for said preventing of saidoccurrence of denitriding during finish annealing; finish annealing saidsteel sheet coated with said annealing separator thereby providing asteel sheet having a surface with glass film formation being absent onsaid surface.
 7. A process for producing a grain oriented silicon steelsheet employing an aluminum nitride inhibitor comprising:providing asteel material consisting essentially of, in terms of weight percent,0.8 to 4.8% of Si, 0.012 to 0.05% of acid soluble Al, 0.01% or less ofN, 0.02 to 0.3% of Mn, and 0.005 to 0.040% of S with the balance beingessentially Fe and unavoidable impurities; forming said steel materialinto a steel sheet, said steel sheet having a surface, including coldrolling said steel sheet one or more times, with intermediate annealingeffected between cold rollings if there is more than one cold rolling;after cold rolling, subjecting said steel sheet to decarburizationannealing; coating an annealing separator on the surface of said steelsheet wherein said annealing separator is at least one member selectedfrom the group consisting of Al₂ O₃, SiO₂, ZrO₂, BaO, CaO, SrO, andMgSiO₄, whereby said annealing separator is disposed between adjacentsteel sheet surfaces during finish annealing; preventing occurrence ofdenitriding of said steel sheet during finish annealing by enrichingprior to secondary recrystallization during finish annealing a surfacesegregation element on the surface of said steel sheet, said surfacesegregation element being a member selected from the group consisting ofSn, Sb and Pb, thereby said surface segregation element providing asurface structure means on the surface of said steel sheet for saidpreventing of said occurrence of denitriding during finish annealing;finish annealing said steel sheet coated with said annealing separatorthereby providing a steel sheet having a surface with glass filmformation being absent on said surface.
 8. A process according to claim6 or 7 further comprising: coating a surface segregation element or acompound of a surface segregation element on the surface of the steelsheet prior to finish annealing.
 9. A process according to claim 8further comprising adding said surface segregation element or saidcompound of said surface segregation element to said annealing separatorprior to coating said annealing separator on said surface of said steelsheet.
 10. A process according to claim 6 or 7 wherein said surfacesegregation element is present in said steel material when said steelmaterial is in a molten state prior to forming said steel sheet.
 11. Aprocess according to claim 1, 2, 4, 5, 6 or 7 further comprisingremoving an oxide layer formed during decarburization annealing from thesurface of said steel sheet prior to finish annealing.
 12. A processaccording to claim 11 or 2, 4, 5, 6 or 7 further comprising coating apowder of at least one member selected from the group consisting of Al₂O₃, SiO₂, ZrO₂, and Mg₂ SiO₄ in slurry form on the surface of the steelsheet as the annealing separator, said powder having an average particlediameter of 0.5 to 10 μm.
 13. A process according to claim 1, 4, or 6wherein said steel material further consists essentially of 0.02 to 0.3%of Mn and 0.005 to 0.040% of S.